1. Field of the Invention
The present invention relates to a resin molded article which can prevent formation of a flow mark also known as a flow pattern.
2. Background Information
Previously, painting was used in order to give a resin molded article, such as thermoplastic resin, a uniform and aesthetically pleasing appearance like metallic tone and pearl tone having a deep feeling.
However, such painting required cumbersome and complicated steps, such as degreasing and washing; prime-coating, intercoating and overcoating; and dust removing, drying and stoving of the surface of the molded article. Further, painting required facilities and paints, and the steps required a longer cycle time. Thus, painting was mainly responsible for higher costs.
To avoid such problems, molding has been carried out using a thermoplastic resin composition. In this thermoplastic resin composition, a metallic piece, glitter material and a colorant are directly mixed into a synthetic resin. The metallic piece, such as scaly-aluminum powders, acts as an agent which imparts metallic or pearl tone. An example of the glitter material is pearlescent pigment made of mica. A molded article using such a thermoplastic resin composition is referred to as a resin molded article.
As shown in FIG. 2 however, at the molding of the resin molded article 9, a flow mark 8 can occur in some cases at a downstream side of the resin flow, where the downstream side is near a molded article end 92 remote from a gate position 91 at molding. Such a flow mark 8 occurs along with the resin flow in a shape substantially similar to the letter "U", as shown in FIGS. 2 and 3.
Since the flow mark 8 deteriorates the decorative effect of the molded article, the present inventors have studied extensively to prevent the formation of such a flow mark. The present inventors have found that the flow mark is primarily composed of relatively dark portions and relatively light portions, alternating from one to another at a remote location from the gate position, as shown in FIG. 3. The present inventors have further discovered that the flow mark generally is not easily formed in a case where the color of the molded article is concentrated, whereas the flow mark is easily formed where the color of the molded article is pale color or colorless. Moreover, as the amount of the glitter material added increases, the likelihood of the flow mark formation increases also, as conceptionally illustrated in FIG. 4.
The present inventor's study of the mechanism of the formation of the flow mark has revealed that the flowing state at the front portion of the flow becomes unstable due to the fact that the rate of the resin flow is reduced at the downstream side in a cavity near an end of the molded article in the flow direction N, as shown in FIG. 5(A). For this reason, flow distortion 81 occurs causing the breakage 82 at the front portion of the flow. The "dark portion" is formed in the breakage portion 82 where adhesion to the cavity surface is lowered which generates a concave and convex portion on the surface.
As shown in FIG. 5(B), on the surface of the molded article having the flow mark, multiple linear grooves 72 can be observed at the dark portion. On the other hand, at the light portion, such linear groove 72 is not formed. Glitter material 71 is in the interior of the molded article. Also shown in FIG. 5(B) are the flow distortion and breakage at the front portion of the flow, which are formed at the downstream side in the resin flow direction N.
Subsequently, the present inventors have studied adjustment of injection pressure for a thermoplastic resin composition, adjustment of a rate of the resin flow, and other factors in order to prevent the flow mark from occurring. However, the molding conditions were difficult, and thus obtained no sufficient results.